1. Field of the Invention
This invention relates generally to safety pressure relief apparatus, and more particularly, but not by way of limitation, to a safety pressure relief apparatus which includes a reverse buckling rupture disk.
2. Description of the Prior Art
A variety of safety pressure relief apparatus of the rupture disk type have been developed. Generally these devices include a rupture disk supported between a pair of complementary supporting members of flanges which are in turn connected to a relief connection in a vessel or system containing fluid pressure. When the fluid pressure within the vessel or system exceeds the design rupture pressure of the disk, rupture occurs causing fluid pressure to be relieved from the vessel or system.
In recent years, rupture disk assemblies of the "reverse buckling" type have been developed which are capable of operating at 90 percent or more of the pressure at which the disk is designed to rupture. Such assemblies generally include a rupture disk having a concave-convex portion connected to an annular flat flange portion by a curved transition connection and a pair of annular supporting members between which the annular flat flange portion of the rupture disk is clamped. The rupture disk is positioned in the assembly so that fluid pressure from the system or vessel to be protected is exerted on the convex side of the disk. This results in the disk being placed in compression during operation and allows the normal fluid pressure exerted on the disk to be relatively close to the pressure at which the disk is designed to rupture.
In order to open the disk and to prevent the formation of loose pieces upon rupture, reverse buckling rupture disk assemblies have heretofore included knife blades upon which the disk impales when reversed by excess fluid pressure. One such knife blade assembly is described in U.S. Pat. No. 3,294,277 to Wood which is assigned to the assignee of this present invention. In addition, reverse buckling rupture disks have been developed and used which include scores or grooves on a surface of the concave-convex portion of the disk creating lines of weakness therein so that upon reversal, the concave-convex portion tears along the lines of weakness and opens without loose pieces being formed. Such a scored reverse buckling rupture disk is described in U.S. Pat. No. 3,484,817 to Wood, assigned to the assignee of this invention.
Rupture disks have also been developed using serrated cutting blades. Russian Pat. No. 396,515 to Malakhov et al. shows a concave-convex portion in combination with an X-shaped serrated cutting blade. U.S. Pat. No. 2,095,828 to Nerad shows a flat rupture disk and a circular serrated cutting member.
In the heretofore used reverse buckling rupture disk apparatus including knife blades, the knife blades are usually arranged so that upon reversal, the concave-convex portion of the rupture disk is cut into quarters or petals which are folded outwardly by the force of fluid under pressure flowing therethrough. Such knife blades are costly to manufacture and are usually positioned in a separate part of the rupture disk assembly making the entire assembly costly to manufacture. In addition, problems have been encountered in the operation of reverse buckling rupture disk assemblies including knife blades due to the knife blades becoming dull from corrosion and/or repeated usage and failing to sever the rupture disk upon reversal whereby pressure relief is not accomplished, or only partial severing of the rupture disk results whereby full opening is not achieved.
With the development and use of scored reverse buckling rupture disks, the problems associated with the use of knife blades were overcome. However, problems have also been encountered in the use of scored reverse buckling rupture disks in that such disks at low design pressures sometimes reverse erratically whereby full opening is not achieved or reverse without rupturing. Also, if such disks are damaged, i.e., accidentally deformed, during handling or installation, reversal without rupture can occur. Unless the disks are manufactured in a manner whereby the ratio of tension rupture pressure to design reversal rupture pressure is low, the fluid pressure required to rupture the disks in tension after reversal without rupture can exceed the design reversal rupture pressure of the disks by a considerable amount creating an extremely dangerous over-pressure condition in the vessels or system intended to be protected.
The term "tension rupture pressure" is used herein to mean the fluid pressure exerted on the concave-convex portion of a reverse buckling rupture disk required to cause the rupture thereof after the concave-convex portion has reversed but not ruptured due to damage or other reason. The term "design reversal rupture pressure" is used herein to mean the fluid pressure exerted on the convex side of a reverse buckling rupture disk at which the concave-convex portion thereof is designed to (and does under normal conditions) reverse itself and rupture.
Since under applicable pressure vessel and pipe codes, the test pressure of pressure vessels and systems is set at 1.5 times the design pressure of such vessels and systems, it is desirable that reverse buckling rupture disks have a design reversal rupture pressure at or below the design pressure of the vessel or system to be protected and a tension rupture pressure no higher than 1.5 times the design reversal rupture pressure. This insures that if such disks reverse without rupturing, rupture will ultimately occur before the pressure level within the vessel or system exceeds the test pressure thereof.
This ratio of tension rupture pressure to design reversal rupture pressure is referred to herein by the term "damage ratio". This term is derived from the fact that reversal without rupture is generally the result of the rupture disk being damaged during handling or installation. The damage ratio is therefore the ratio of the pressure at which a damaged disk will rupture to the pressure at which an undamaged disk will rupture.
In applications for scored reverse buckling rupture disks wherein the design reversal rupture pressure is in a normal pressure range, i.e., above about 125 psig in 1 inch size to above about 40 psig for 6 inch size, scored reverse buckling rupture disks have been developed and used successfully wherein the damage ratio is 1.5 or less. However, in low pressure applications, the disks are difficult to manufacture and the damage ratio is more difficult to control resulting in a possibility that the test pressure of the vessel or system being protected will be exceeded before rupture occurs, and because of the low fluid pressure, reversal of a scored reverse buckling rupture disk can more readily take place without sufficient force being exerted on the disk after reversal to cause it to fully tear along the lines of weakness created by the scores thereon resulting in only partial opening. Thus, the use of scored reverse buckling rupture disks has been limited to applications wherein the design reversal rupture pressures thereof are within the normal pressure range mentioned above.
In most reverse buckling rupture disk apparatus developed and used heretofore, problems have been encountered which result from uncontrolled reversal of the disks. That is, nearly all of the reverse buckling rupture disks utilized heretofore include a concave-convex portion connected to an annular flat flange portion by a curved transition connection. With the exception of certain designs which reverse from the center portion outwardly, e.g., scored reverse buckling rupture disks and others including weakened center portions, when excess fluid pressure is exerted on such disks, the reversal process starts at the transition connection. That is, the transition connection is moved inwardly toward the center of the disk at a point thereon having the least resistance followed by the reversal of the entire concave-convex portion of the disk. Heretofore, the particular point along the transition connection at which the reversal starts has been left to chance, sometimes resulting in less than desirable operational results. That problem is avoided in the present invention by providing a portion of said transition connection with a greater radius of curvature so that reversal will initiate at that portion of greater radius of curvature.
By the present invention, a safety pressure relief apparatus of the reverse buckling rupture disk type is provided which is economical to manufacture and which includes a knife blade for partially severing the disk upon reversal, but which obviates the problems relating to knife blades mentioned above. Further, the apparatus of the present invention achieves full opening in extremely low pressure applications while maintaining a damage ratio of 1.5 or less. Also, the reversal process of the rupture disk of this invention is controlled in a manner whereby the partially severed portion of the rupture disk formed upon reversal is retained and prevented from being moved downstream with the fluid being relieved.
A safety pressure relief apparatus somewhat similar to that of the present invention is disclosed in U.S. patent application Ser. No. 832,417 to Witten et al., assigned to the assignee of the present invention. Witten et al. discloses a completely circular cutting edge which completely severs a substantially circular portion of the rupture disk, which severed portion is then caught on a curved catcher portion spaced downstream from the cutting edge.
The present invention provides a preferable alternative to Witten et al., in that the rupture disk is only partially severed and is retained by an integral hinge portion of the rupture disk, thereby eliminating the possibility that the severed portion might miss the catcher bar and be transported downstream by the escaping fluid.
An additional improvement provided by the present invention is the rounded notches between adjacent teeth of the serrated cutting edge. Using a preferred extension of the rounded notch concept, the teeth may be formed by cutting overlapping circular portions from the support member so that the teeth are defined by intersecting circular arc notches. These rounded notch designs permit the use of fewer larger teeth which extend a shorter radially inward distance. These shorter larger teeth are also stronger and less easily deformed. The use of shorter teeth increases the flow area of the opening of the support member and also causes the rupture disk to be severed at relatively lower pressures, since there is a greater stress concentration at the point of each tooth for a given differential.